Stereoisomers are those molecules which differ from each other only in the way their atoms are oriented in space. Stereoisomers are generally classified as diastereomers or enantiomers; the latter embracing those which are mirror-images of each other, the former being those which are not. The particular arrangement of atoms that characterize a particular stereoisomer is known as its optical configuration, specified by known sequencing rules as, for example, either + or − (also D or L) and/or R or S.
Though differing only in orientation, the practical effects of stereoisomerism are important. For example, the biological and pharmaceutical activities of many compounds are strongly influenced by the particular configuration involved. Indeed, many compounds are only of widespread utility when employed in a given stereoisomeric form.
Living organisms usually produce only one enantiomer of a pair. Thus only (−)-2-methyl-1-butanol is formed in yeast fermentation of starches; only (+)-lactic acid is formed in the contraction of muscle; fruit juices contain only (−)-malic acid, and only (−)-quinine is obtained from the cinchona tree. In biological systems, stereochemical specificity is the rule rather than the exception, since the catalytic enzymes, which are so important in such systems, are optically active. For example, the sugar (+)-glucose plays an important role in animal metabolism and is the basic raw material in the fermentation industry; however, its optical counterpart, or antipode, (−)-glucose, is neither metabolized by animals nor fermented by yeasts. Other examples in this regard include the mold Penicillium glaucum, which will only consume the (+)-enantiomer of the enantiomeric mixture of tartaric acid, leaving the (−)-enantiomer intact. Also, only one stereoisomer of chloromycetin is an antibiotic; and (+)-ephedrine not only does not have any drug activity, but it interferes with the drug activity of its antipode. Finally, in the world of essences, the enantiomer (−)-carvone provides oil of spearmint with its distinctive odor, while its optical counterpart (+)-carvone provides the essence of caraway.
Thus, as enzymes and other biological receptor molecules possess chiral structures, enantiomers of a racemic compound may be absorbed, activated, and degraded by them in different manners. This phenomenon causes that in many instances, two enantiomers of a racemic drug may have different or even opposite pharmacological activities. In order to acknowledge these differing effects, the biological activity of each enantiomer often needs to be studied separately. This and other factors within the pharmaceutical industry have contributed significantly to the need for enantiomerically pure compounds and thus the need for chiral chromatography.
Accordingly, it is desirable and oftentimes essential to separate stereoisomers in order to obtain the useful version of a compound that is optically active.
Separation in this regard is generally not a problem when diastereomers are involved: diastereomers have different physical properties, such as melting points, boiling points, solubilities in a given solvent, densities, refractive indices etc. Hence, diastereomers are normally separated from one another by conventional methods, such as fractional distillation, fractional crystallization or chromatography.
Enantiomers, on the other hand, present a special problem because their physical properties are identical. Thus they cannot as a rule—and especially so when in the form of a racemic mixture—be separated by ordinary methods: not by fractional distillation, because their boiling points are identical; not by conventional crystallization because (unless the solvent is optically active) their solubilities are identical; not by conventional chromatography because (unless the adsorbent is optically active) they are held equally onto the adsorbent. The problem of separating enantiomers is further exacerbated by the fact that conventional synthetic techniques almost always produce a mixture of enantiomers. When a mixture comprises equal amounts of enantiomers having opposite optical configurations, it is called a racemate; separation of a racemate into its respective enantiomers is generally known as a resolution, and is a process of considerable importance.
Chiral columns that can resolve a large number of racemic compounds (general chiral columns) are in high demand. They are needed routinely in many laboratories, especially in pharmaceutical industry. Prior to the present invention, Daicel columns, macrocyclic antibiotic columns, and the Whelk-O columns were probably known as the industrial leaders in this type of general chiral columns. The present inventor has developed a new class of general chiral columns based on the use of proline and its analogues.
Furthermore, and importantly, the columns of the present invention have the capability of resolving at least a similar or higher percentage of the compounds tested. Furthermore, the columns of the present invention provide better separation on some of the compounds tested and can resolve certain compounds that cannot be resolved with the commonly used commercial columns listed above.
The columns of the present inventions are stable and can be used with a large number of mobile phase solvents. Therefore, the columns of the present invention should find important applications as general chiral columns.
As stated above, a large number of chiral columns have been prepared in the past; however, only a few demonstrated broad chiral selectivity. The successful examples include the popular Daicel columns, the Chirobiotic columns, and the Whelk-O½ columns. The Daicel columns are prepared by coating sugar derivatives onto silica gel. Chirobiotic columns are prepared by immobilizing macrocyclic glycopeptides onto silica gel. Whelk-O ½ columns contain both electron rich and electron deficient aromatics. These columns have broad chiral selectivity and have been applied successfully to resolve a fair number of racemic compounds. They have different selectivity and stability profiles. Their selectivities complement each other in some cases, while they duplicate each other in other cases. Some of the columns are more suited for reversed phase conditions and others for normal phase conditions. Each column has its own strengths and weaknesses. Despite these progresses, there are still many compounds that cannot be resolved or resolved well using these commercial available columns. Therefore, there is still a significant need to develop new columns that have relatively broad chiral selectivity.